A lunar month, the length of time between new moon and new moon, is about 29.5 days. 12 lunar months make 354 days, some 11 days short of a solar year. Without correction, a purely lunar calendar will slip behind the seasons very rapidly, and cycle through all of them in 30 years. This is how the current Muslim calendar operates.
Other cultures, however, have sought to correct this problem by adding an extra month every few years, eventually refined to 7 extra (intercalary) months every 19 years, which is very close to a solar reckoning. This is how many ancient calendars worked, and the Jewish calendar still does so today. The Christians inherited from the Jews a similar system for calculating the date of Easter, since Easter is historically tied to the Jewish Passover festival.